For the mechanism, please refer to the text on 1,3-dipolar cycloaddition. This reaction is highly exothermic, but the high activation barrier is responsible for a very low reaction rate, even at elevated temperature. Another drawback is the formation of regioisomers, as the two possible HOMO-LUMO interactions of the substrates are closely related in terms of energy. The thermal reaction therefore often gives approximately 1:1 mixtures of both the 1,4-substituted and the 1,5-substituted regioisomers.